Do not remove feature flag in teardown in add_remove_ci_variable_spec

What does this MR do?

Do to remove feature flag in teardown in add_remove_ci_variable_spec Also, add requries_admin tag as this test enables a feature flag

On live environments, if two instances of this test are running and they both try to change the feature flag, this could lead to inconsistencies and flakiness.

Related to #214461 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content